In the past year, our efforts focus on the role of SIRT1 in intestinal tissue homeostasis, the function of SIRT1 phosphorylation in tissue-specific energy metabolism, as well as the role of SIRT1 in ES cell biology and animal development. As a highly conserved NAD+-dependent protein deacetylase, SIRT1 has been shown as a key metabolic sensor that directly links nutrient signals to animal metabolic homeostasis. Although the functions of SIRT1 have been extensively studied in various metabolic tissues in recent years, the role of SIRT1 in nutrient absorption and sensing in small intestine, a key metabolic organ that provides the first interface between nutrients and animal metabolism, is still completely unknown. To elucidate the function of SIRT1 in intestinal metabolism, we recently generated a novel intestine-specific SIRT1 KO mouse model, SIRT1 IKO mice and showed that intestinal SIRT1 is an important regulator of ileal bile acid absorption that feedback modulates systemic bile acid homeostasis and cholesterol metabolism (Kazgan et al, Gastroenterology, 2014). In the past year, we further worked on the role of SIRT1 in intestinal tissue homeostasis. We discovered that deletion of SIRT1 specifically in intestinal epithelial cells stimulates cellular NF-kB signaling and Nrf2-mediated oxidative stress pathways, leading to hyperactivation of Paneth cells, enhanced intestinal inflammation, and altered gut microbiota. Under stress conditions induced by nutrients, chemicals, or aging, the extensive interaction between these three components increases the susceptibility to colitis. Importantly, we discovered that the mRNA levels of human SIRT1 were significantly decreased in human ulcerative colitis patients. Together with a recent report that a SIRT1 mutation that reduces its deacetylase activity is associated with colitis in humans, our findings support the hypothesis that SIRT1 is a key genetic factor that regulates the susceptibility to inflammatory bowel diseases (IBDs) in both humans and mice. This study uncovers a new role for SIRT1 in intestinal tissue homeostasis, and suggests that small molecules that activate SIRT1 maybe beneficial for treatment of human IBDs. A manuscript describing this exciting study is currently under review. In addition to colitis, we recently found that intestinal SIRT1 also plays an important role in the regulation of colon cancer in a dose-dependent manner. Heterozygous deletion of SIRT1 induces colon cancer formation, whereas homozygous deletion of SIRT1 triggers cellular apoptotic pathways, leading to reduced cancer formation. In collaboration with researchers at UNC-Chapel Hill, we further discovered that increased expression of SIRT1 in colon cancer is associated with increased survival after surgery in human colon cancer patients. This study highlights the importance of SIRT1 in dose-dependent regulation of tumorigenesis. A manuscript describing this study is under review. SIRT1 is also a key regulator of animal development. However, despite the fact that the developmental defects of the first SIRT1 KO mouse model was reported more than 10 years ago, the molecular mechanisms underlying this important function of SIRT1 remain largely unclear. Using embryonic stem cells (ESCs) and mice as models, we recently discovered that SIRT1 contributes to the maintenance of homeostatic retinoic acid (RA) signaling and modulates mouse ESC differentiation. We further found that SIRT1 deficiency induced developmental defects is associated with elevated RA signaling in mice (Tang et al., Molecular Cell, 2014). In the past year, we continued our efforts in understanding the function of SIRT1 in stem cell biology and animal development, with focus on dissecting molecular mechanisms underlying SIRT1 deficiency induced developmental defects. Two manuscripts related to this direction are currently in preparation.